Piston



FIG. 1 is an elevational view of one end of a piston, showing our new design, the opposite end being a mirror image thereof;

FIG. 2 is an elevational view of one side thereof, the opposite side being a mirror image;

FIG. 3 is a bottom plan view thereof;

FIG. 4 is a top plan view thereof. 

The ornamental design for a piston, as shown and described. 